The present invention relates to an optical fiber coupler and a method of producing the same and, more particularly to an optical fiber coupler which is manufactured using an optical fiber whose surface is coated with carbon and has high hydrogen resistance and high static fatigue strength, and a method of producing the same.
An optical communication system incorporates optical fiber couplers which have a function to branch and join light having a single wavelength or to separate and combine light having plural wavelengths.
Recently, optical fiber couplers have been used in a wide variety of applications including submarine optical cable communication and automotive intercommunication systems. As the applications expand, optical fiber couplers are demanded more strongly than before to have higher hydrogen resistance and higher static fatigue strength.
A conventional optical fiber coupler A, which has been used in optical communication systems, has a construction as shown in FIG. 1, in which at a predetermined location in the lengthwise direction of two optical fibers 1 and 2, the coatings 1a and 2a formed of, for example, ultraviolet-curing resin on the outer periphery of respective clads are removed, the exposed clad portions are welded and extended by heating them by means of, for example, a micro torch, and the cores of the optical fibers 1 and 2 are approached to each other to the extent that evanescent bonding can be performed in order to form a connected portion 3.
As shown in FIG. 2, this optical fiber coupler A is contained in an enclosure 5 with the connected portion 3 being fixed onto a reinforcing plate 4 consisting of, for example, quartz with an adhesive, and the enclosure 5 is filled with, for example, a resin to prevent moisture from entering the connected portion 3 from the outside for practical use.
In the optical fiber coupler A, part of the light inputted to the core of, for example, optical fiber 1, branches off to the core of the optical fiber 2 in a predetermined branching ratio at the connected portion 3. Thus, each light is transmitted and outputted through respective cores of the optical fibers 1 and 2.
Such an optical fiber coupler has the following great problems in practical use: The hydrogen resistance of the connected portion 3 should be increased so that a high mechanical strength is maintained for a long period of time and high reliability is ensured. Also, the static fatigue strength of the optical fiber itself should be increased so that the design strength is kept for a long period of time and the portion extending from the enclosure can be bent to a small radius of curvature for use. In particular, the high static fatigue strength is a useful feature for miniaturizing the system size including the optical fiber coupler.
A variety of methods have so far been used to manufacture optical fiber couplers. For example, Japanese Unexamined Patent Publication No. 60-107606 discloses one method as described below.
With this method, a metal coated optical fiber whose clad surface is coated with a metal such as Al or Ni is used as an optical fiber.
First, a desired length of metal coating layer of the metal coated optical fiber is dissolved and removed by using, for example, solution of hydrochloric acid to expose the clad surface, and then the metal coat removal portion is rinsed. Two metal coated optical fibers thus treated are put in parallel with their metal coat removal portions being in contact with each other. Then, two metal coat removal portions are welded by heating by means of, for example, oxygen-hydrogen flame to form a connected portion. In this process, two metal coated optical fibers may be pulled so that the metal coat removal portion is extended to an extent that a desired diameter can be obtained.
In this process, the branching ratio of light at the formed connected portion can be set appropriately by performing the aforementioned welding work while inputting light from one metal coated optical fiber and measuring the light outputted from the other metal coated optical fiber.
However, the optical fiber coupler manufactured by this method has the following problems: First, the metal coating layer must be pickled and then rinsed to form the metal coat removal portion as a preliminary process before the formation of connected portion. In this process, the clad (glassy material) inevitably comes into contact with water. As a result, the strength of the optical fiber at the metal coat removal portion is reduced, so that it is difficult to form a connected portion with a high strength. Although it is advantageous from the industrial viewpoint that the processes of pickling and rinsing are omitted and two optical fibers are directly welded and extended, this is impossible with the above method.
Japanese Unexamined Patent Publication No. 2-127604 discloses another method for manufacturing an optical fiber coupler as described below.
With this method, part of the coating of an optical fiber is removed, and two optical fibers whose clad is exposed are brought into contact with each other. While light is inputted from one optical fiber and the light outputted from the other optical fiber is measured, the portion in contact is heated and welded, and at the same time, the optical fiber is extended in the optical axis direction. When the measured light exhibits a predetermined branching ratio, the melting/extending operation is stopped to form an extended portion (connected portion). After that, the extended portion is coated with a carbon film by means of, for example, incompletely burned gas of acetylene.
However, this method has a problem in that the carbon film formed at the extended portion functions as an absorbing layer of light. Specifically, the extended portion, which is formed so as to have a predetermined branching ratio in the welding/extending operation, exhibits another branching ratio when the carbon film is formed on it.
In both of the techniques disclosed by Japanese Unexamined Patent Publication No. 60-107606 and Japanese Unexamined Patent Publication No. 2-127604, if the clad surface of glassy material is damaged in forming the connected portion, it is necessary to perform flaming or hydrofluoric acid polishing so as to ensure smooth welding of clads.